VM: Make Dart::FeaturesString respect isolate flags controlling asserts and type checks.

runtime/vm/kernel_isolate.cc

Index: runtime/vm/kernel_isolate.cc
diff --git a/runtime/vm/kernel_isolate.cc b/runtime/vm/kernel_isolate.cc
index e10391cdb932a78d8b5af423750ebcdb36eb3229..f14b99eccd0ef9c286f9ed8924c8b7bef96c454d 100644
--- a/runtime/vm/kernel_isolate.cc
+++ b/runtime/vm/kernel_isolate.cc
@@ -69,7 +69,11 @@ class RunKernelTask : public ThreadPool::Task {
     }
 
     Dart_IsolateFlags api_flags;
-    Isolate::FlagsInitialize(&api_flags);
+    api_flags.version = DART_FLAGS_CURRENT_VERSION;
+    api_flags.enable_type_checks = false;
+    api_flags.enable_asserts = false;
+    api_flags.enable_error_on_bad_type = false;
+    api_flags.enable_error_on_bad_override = false;
 
     isolate = reinterpret_cast<Isolate*>(create_callback(
         KernelIsolate::kName, NULL, NULL, NULL, &api_flags, NULL, &error));